This invention relates to calligraphic image generation finding particular application in aircraft flight simulators. While the invention is described with particular emphasis on its flight simulator application aspects, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
Flight simulators are well known in the art. A device with which the present invention is particularly well adapted for use is shown and described in U.S. Pat. No. 3,996,673 to Carl J. Vorst et al, issued Dec. 14, 1976, the disclosure of which is specifically incorporated by reference herein. As discussed in U.S. Pat. No. 3,996,673, it is generally advantageous to perform computations using digital techniques. The advantages of digital processing are well recognized in the art, and are not discussed in detail. It is sufficient here to note that digital computations simplify hardware. Such hardware has become increasingly cost competitive with corresponding analog methods for accomplishing similar objectives. A major problem with digital processing in calligraphic image generation, however, is that encountered in converting the digital signal accurately back to a smooth analog wave form required for data presentation on a CRT, for example. Digital-to-analog converters are step output devices. If the output wave form rate of change is slow enough, the step size can be made insignificant by using a high conversion rate. If the rate of change is high, however, minimum step size becomes limited by maximum digital-to-analog converter update rate.
Present devices for smoothing digital-to-analog conversion steps involve either low pass filtering or passing the output of the digital-to-analog device through a notch filter having a center frequency the same as that of conversion. In general, these devices result in a non-linear response to a step input, which is not a desirable output if the intended use of the output is a continuous ramp. Certain prior art also is known which utilize for other purposes certain structural features found in the present invention. For example, U.S. Pat. No. 3,543,009 to Voelcker, issued Nov. 24, 1970, shows, in FIG. 2, an analog transversal filter (AFT). The filter includes a tapped delay line with an analog summation of the tap outputs. As recognized in the '009 patent, complicated analog LTI filters are essentially fixed rate wave form generators. If one wants to change the signalling rate significantly, i.e., to scale in time, it is often cheaper to provide new filters rather than to incorporate adjustment features. The '009 patent approached the solution to this problem by utilizing shift registers in which the input pulse units are shifted through the stages of the shift register at a rate faster than the fastest rate of occurrence of such pulse units. My invention, on the other hand, overcomes the shortcomings of prior art ATF's, by incorporating means for varying the effective sampling rate of the device. Consequently, I can employ more conventional delay lines compatible, for example, with the image generating device disclosed in the above-referenced Vorst U.S. Pat. No. 3,996,673.
U.S. Pat. No. 3,317,845 to Terp, issued May 2, 1967, employs a tapped delay line as part of a matched filter. The filter is designed to pass low level signals effected only by the characteristic response of the delay line. If the signal input exceeds some threshold level, the output is gated off for a period equal to one half of the expected input pulse width, then momentarily gated on, then off again. The circuit operates to convert a radar pulse and its sideband reflections to a single pulse. The delay line is used as a timing device for gate control in filter operation.
As disclosed hereinafter, a tapped delay line is utilized with a summing network as part of my invention. However, the invention makes use of gates to control the period of time in which the input signal is summed. The time period is matched to the update rate of a digital input signal. Means also are included to varying summing gain of the interpolator of this invention.
The invention disclosed hereinafter has the advantage of producing a linear ramp output, given a step input. If the digital data updates are spaced by a time period T, the interpolator may be configured to produce a continuous ramp output.
One of the objects of this invention is to provide a device for providing a continuous output wave from a stepped input wave.
Another object of this invention is to provide a low cost device for providing a continuous output wave from a stepped input wave.
Another object of this invention is to provide a device for controlling the period of time in which the input signal is summed.
Another object of this invention is to provide a device in which the time period in which the input signal is summed is matched to the update rate of a digital signal input to the device.
Another object of this invention is to provide a device which may accommodate a varying input signal update rate by controlling the number of gates activated from the delay line along with varying the summing gain of a summing device.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.